Two-stage turbochargers are commonly known and are used in all kinds of engines. They consist of a high-pressure (HP) turbine, and a low-pressure (LP) turbine, with each turbine having its own compressor. During normal operating conditions, when the engine runs at lower speeds, the only turbine typically in use is the HP turbine. When the engine is running at lower speeds, it creates less exhaust gas energy. This lower amount of exhaust gas energy is typically not enough to power the LP turbine, but it does provide enough energy to power the HP turbine. During operation, as the engine begins to increase speed, the HP turbine is typically operated by the lower energy exhaust gases, but after the engine reaches a certain speed and load, the HP turbine begins to operate in series with the LP turbine until the HP turbine provides enough flow capacity to have any effect on engine performance. When this occurs, the LP turbine begins to operate and generate the higher level of boost pressure than the HP turbine cannot generate in series with the LP turbine. Increasing engine speed also increases the exhaust gas energy, which is necessary to operate the LP turbine.
Another common problem with two-stage turbochargers occurs at higher engine speed, when the HP turbine is not cut off from the air flow of the exhaust gas. During this condition there is the possibility of “overspeed,” i.e., the increased exhaust gas energy can cause the HP turbine to spin at speeds which may cause damage. Control valves of two-stage series turbocharger systems have been applied to modulate the amount of exhaust gas pressure flowing into the LP turbine. These valves typically have been used for closing off exhaust gas flow to the LP turbine thereby only allowing the exhaust gas to flow only to the HP turbine until the HP turbine is no longer effective, at which point the valve opens a pathway to allow exhaust to flow to the LP turbine. This is beneficial in providing boost pressure at low engine speeds, but does not aid preventing overspeed of the HP turbine.
Accordingly, there exists a need for an improvement in transitioning from the HP turbine to the LP turbine in a two-stage turbocharger system, as well as an improvement in the prevention in overspeed in a HP turbine.